The present disclosure is directed to an adjustable seal apparatus for mounting a mixing apparatus or other devices having a movable shaft in sealed engagement with a wall of a vessel, and in particular to an adjustable seal apparatus for adjustably mounting the mixing apparatus to the vessel in a selected orientation with respect to the vessel while maintaining a seal between the mixing apparatus and the vessel. The adjustable seal apparatus includes a seal mechanism for creating a gas-tight liquid seal between the mixing apparatus and the wall of the vessel, and an adjustable mounting mechanism for mounting the mixing apparatus to the wall of the vessel in desired orientation with respect to the wall of the vessel while the seal mechanism maintains the gas-tight liquid seal between the mixing apparatus and the wall of the vessel.